I changed the SNR Margin so it's now 12db, nevertheless it seems that these slowdowns for a few minutes to just a few kbps persist. I really have no idea what can be causing it?
Is there any merit to changing my ISP, I'm currently with Demon?
If throughput stops completely for a few moments and then recovers, then it may be a deficiency of your router, wiring or line, as the slowdown might coincide with a drop in connection and/or a burst of errors. But if it simply slows to a few kbps, which lasts for some time, it is more likely to be traffic management, or simple congestion, within your ISP. In that case, tweaking the target noise margin won't help in the slightest.
You could install routerstats and monitor your line to try and prove it one way or the other, but the fact you are with demon makes me deeply suspicious. Demon have long-since had a traffic shaping policy, they have also recently adopted a tactic of 'prioritising' business traffic. In other words those who pay more get better throughput. But there's only so much bandwidth in the melting pot so, in order to create the bandwidth to 'prioritise' business users, it would appear that they must be wilfully impeding traffic from other subscribers. I am also with demon, and it is not at all unusual for throughput to drop to modem speeds (say, 50kbps) at busy times of day.
Personally, I am unable to escape from demon as many websites (some of which I care about) still link to my old ----demon.co.uk website, despite the fact I changed it to a non-demon ---.com address many years ago. Similarly, people still send mail to my ----.demon.co.uk address. despite having long since got my own ----.com. If it were not for that, I'd have dropped them like a hot potato years ago. That is, of course, just my opinion.